1. Field of the Invention
The present invention relates to an apparatus and method for controlling the power of a laser emitted to an optical recording medium in order to record information to or reproduce information from an optical recording medium.
2. Description of Related Art
Optical disc drives for reading and writing information to optical storage media accomplish the reading and writing operations by emitting a laser beam to the optical disc. A digital operation is used to control the power of the laser beam emitted to the disc.
Laser power control in a typical optical disc drive is described next with reference to FIG. 5. FIG. 5 is a block diagram of a conventional laser power controller that controls laser power using a digital process.
As shown in FIG. 5, this laser power controller has an operating circuit 101, A/D converter 115, D/A converter 125, laser drive current source 126, and semiconductor laser 110. The laser power controller also has a semiconductor laser 111 which is a pin photodiode and monitors the power of the laser beam output from semiconductor laser 110, and a current-voltage converter 112 for converting the monitor current from pin photodiode 111 to a voltage. D/A converter 125 has a D/A converter (DABS circuit) 104 for a bias power current, and a D/A converter (DAPK circuit) 105 for a peak power current.
This laser power controller sets laser power as follows.
A recording power learning process is first conducted. Recording power is learned by the operating circuit 101 gradually increasing output to DABS circuit 104 in D/A converter 125 while controlling DABS circuit 104 so that A/D converter 115 output is adjusted to the two specific values ADBSa and ADBSb needed to achieve the bias power (erase power) level, and then obtaining the drive current values DABSa and DABSb at which ADBSa and ADBSb are achieved. These two values ADBSa and ADBSb are then set as the near upper and lower limits of the range containing the desired bias power level.
Slope Kb near bias power Pb can be obtained from the following equation.
Kb=(DABSbxe2x88x92DABSa)/(ADBSbxe2x88x92ADBSa)
Next, while DABS circuit 104 of D/A converter 125 is holding DABSb and output to DAPK circuit 105 of D/A converter 125 is gradually increased, DAPK circuit 105 of D/A converter 125 is controlled so that output of A/D converter 115 goes to the two specific values ADPKa and ADPKb needed to achieve peak power (recording power). Drive current values DAPKa and DAPKb corresponding to ADPKa and ADPKb obtained at that time are detected. These two values ADPKa and ADPKb are set as the near upper and lower limits of the range containing the desired peak power level. The slope near peak power Pp can be obtained from the following equation similar to the above bias power equation.
Kp=(DAPKbxe2x88x92DAPKa)/(ADPKbxe2x88x92ADPKa)
It is assumed that the bias power Pb and peak power Pp required for recording are A/D converter 115 output values ADBSX and ADPKX, respectively. DABSX and DAPKX output from operating circuit 101 to DABS circuit 104 and DAPK circuit 105 of D/A converter 125 to achieve bias power Pb and peak power Pp can be obtained from the following equations.
DABSX=Kb*(ADBSXxe2x88x92ADBSa)+DABSa
DAPKX=Kp*(ADPKXxe2x88x92ADBSXxe2x88x92(ADPKaxe2x88x92ADBSb))+DAPKa.
This is further described in, for example, Japanese Patent Laid-Open Publication No. 6-338073.
Problems with this control method are described next. When reproduction continues for a long time after recording is completed, the surrounding temperature changes. For a semiconductor laser, the relationship between laser power and drive current changes as the temperature changes. This means that the recording power changes when recording starts again, thus inviting deterioration in recording reliability. Therefore, recording power learning is necessary to avoid this.
On the other hand, since recording requests occur at random and the seek time needed to find a recording area tends to increase as recording volume increases, out-of-focus power learning and power learning in a separate learning area as a means of shortening the time to the start of recording also degrade disc access performance, and cannot be easily implemented.
Current output to the laser generating section at the start of recording is therefore set to the current setting of the previous recording operation, or to a current setting determined when the power turns on or as a result of an adjustment process. The current needed for driving a semiconductor laser to emit at a particular power level changes according to the temperature. The current required for the laser to output at a specific power level is therefore temperature dependent. This means that if the temperature of the semiconductor laser at the end of one recording operation differs greatly from the laser temperature at the start of a next recording operation, actual laser output power at the next recording operation may differ greatly from the required recording power.
A common means of reducing the beam spot size in conjunction with the increase in recording density has been to use only the primary beam of the light output. Thus, both semiconductor laser output power and efficiency at the pick-up power become reduced, and recording power becomes near the rated laser power. This means that the temperature dependence of semiconductor laser output may cause laser output to exceed the rated power, and thus shorten laser life.
Methods for controlling the current used to drive the semiconductor laser based on temperature data alone are also possible. In this method, however, it is also necessary to consider external factors other than temperature change, including changes in IC output resulting from power supply fluctuation and the temperature characteristics of the IC device, and it is difficult to achieve laser power control with the required precision.
The present invention is directed to the aforementioned problems, and provides a laser power control apparatus and method capable of maintaining stable recording power output during data recording even when the temperature around the semiconductor laser changes.
Laser power control according to the present invention detects the temperature difference between the previous recording operation and the present recording time, calculates the value of a drive current setting signal applied to the laser generating means when recording starts again based on the detected temperature difference, and drives the laser according to this drive current setting signal. The relationship between laser drive current and output power is then determined while detecting laser output power, and feedback control is used to adjust the drive current setting signal so that the desired recording power is obtained. This reduces abnormal laser emissions and makes it possible to control the semiconductor laser to output at a stable recording power level even when the output power tends to vary as a result of external factors other than the temperature of the ambient environment.
In the first aspect of the invention, an apparatus for controlling a laser power comprises a laser generator, a laser driver, an operating section, a temperature detector, a memory, and a current variation determination section.
The laser generator emits a laser beam with which data are recorded to or reproduced from an optical disc.
The laser driver controls emission power of the laser generator with a drive current supplied to the laser generator.
The operating section calculates at a first timing a drive current setting signal which controls the drive current supplied from the laser driver, updates the drive current setting signal, and outputs the updated drive current setting signal to the laser driver.
The temperature detector detects temperature of the laser generator.
The memory stores temperature of the laser generator at a predetermined timing and value of the drive current setting signal at the predetermined timing.
The current variation determination section determines variation rate of the value of the drive current setting signal according to a first temperature and temperature difference between the first temperature and a second temperature.
The operating section inputs temperature of the laser generator at start of recording from the temperature detector, reads out the temperature of the laser generator from the memory, obtains the variation rate with reference to the current variation determination section using the input temperature of the laser generator as the first temperature and the read temperature of the laser generator as the second temperature, and determines the value of the drive current setting signal to be used at the start of recording based on the obtained variation rate and the value of the drive current setting signal stored in the memory.
In the apparatus, the predetermined timing may be when the recording operation terminates, or when the operating section updates the drive current setting signal.
The apparatus may further comprise a power detector for detecting power emitted from the laser generator. In this case once recording operation starts, the apparatus may detects the emitted power every predetermined time by the power detector. The operating section may calculate, based on the detected power, the drive current setting signal so that the emission power is equal to a predetermined recording power.
In the apparatus, the temperature detector may detect temperature of a member through which heat of the laser generator conducts.
In the apparatus, the current variation determination section may comprise a table in which the temperature difference between the first and second temperature corresponds to the value of the drive current setting signal at the first temperature. The table may link the temperature difference with the variation rate of the value of the drive current setting signal of the laser generator at the first temperature.
Alternatively, the current variation determination section may comprise an approximate equation which provides, using high order function, relation between the first temperature, the temperature difference between the first and second temperature, and the variation rate of the value of the drive current setting signal at the first temperature.
In a second aspect of the invention, a method of controlling a laser power control apparatus is provided.
The apparatus comprises a laser generator for emitting a laser beam with which data are recorded to or reproduced from an optical disc, a laser driver for controlling emission power of the laser generator with a drive current supplied to the laser generator, a operating section for calculating at a first timing a drive current setting signal which controls the drive current supplied from the laser driver, updating the drive current setting signal, and outputting the updated drive current setting signal to the laser driver, and a current variation determination section for determining variation rate of the value of the drive current setting signal according to a first temperature and temperature difference between the first temperature and a second temperature.
The method comprises, at predetermined timing, storing temperature of the laser generator and value of the drive current setting signal. The method comprises, at start of recording after the predetermined timing, detecting temperature of the laser generator at start of recording, reading out the stored temperature of the laser generator, obtaining the variation rate from the current variation determination section using the detected temperature of the laser generator as the first temperature and the read temperature of the laser generator as the second temperature, and determining the value of the drive current setting signal to be used at the start of recording based on the obtained variation rate and the stored value of the drive current setting signal.